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First published online March 4, 2005
doi: 10.1242/10.1242/dev.01688


Development 132, 1199-1210 (2005)
Published by The Company of Biologists 2005


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Patterning across the ascidian neural plate by lateral Nodal signalling sources

Clare Hudson and Hitoyoshi Yasuo

Unité de Biologie du Développement (UMR7009), CNRS/UPMC, Station Zoologique, Observatoire Océanologique, 06230 Villefranche-sur-mer, France



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Fig. 1. Cell lineages of the ascidian larval CNS. Cell lineages are indicated as follows: the a-line is coloured red (anterior sensory vesicle precursors) or pink (anterior epidermis and pharynx/neurohypothesis precursors); b-line is green and A-line yellow at the 8-cell stage and light yellow (medial cells) or tan (lateral cells) from the 32-cell stage. Bars connecting two blastomeres on the right side of the drawings indicate sister cell relationship. Small circles in the lateral-A-line precursors on the left side indicate that these blastomeres give rise to the motoneurons; the final position of the motoneurons in the visceral ganglion is also indicated on the drawing of the larvae. At the neural plate stage, only the neural plate is shown and the dark blue ovals represent the secondary muscle lineage.

 


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Fig. 6. Expression of marker genes of lateral neural plate and secondary muscle in embryos in which Nodal signalling has been inhibited. Experimental conditions are shown at the top of the panels and the marker analysed is on the left of the panels. (A-D) Neural plate views of embryos at the late gastrula stage. (A) Expression of Ci-Chordin in the notochord can be seen in control and manipulated embryos. (E,F) Embryos at the early tailbud stage are shown in dorsal (far left) and lateral view. Treated embryos are shown in a lateral view. (G) Panel on the far left is a control cleaving embryo at the neurula stage; the rest are treated with cytochalasin from the 76-cell stage. Arrowheads point to the secondary muscle lineage (A8.16) in the control panel.

 


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Fig. 5. A collection of neural plate markers. At the top of the panels is a schematic drawing of the neural plate when it consists of six rows of cells, with each square representing a neural plate cell. Names of each cell are indicated, and should be prefixed with an `a9.' for a-line cells, `A9.' for A-line cells and `b9.' for b-line cells. I-VI indicate the row number, with Row I the closest to the blastopore, which is at the most caudal position of the neural plate. Colour scheme used is as in Fig. 1. (A-F) For each marker, two stages are shown; when the neural plate has six rows of cells (left) and when the neural plate has seven rows of cells (right). Nuclei are labelled to allow easy identification of individual cells. A schematic drawing of the neural plate with the blastomeres expressing each marker coloured in blue is presented below each embryo.

 


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Fig. 2. Ablation of the right side b6.5 blastomere affects A-line neural patterning. Experimental conditions are shown at the top of the panels and the marker used on the left of the panels. All embryos are in vegetal pole view. Schematic representations of embryos are shown on the left, with black dots indicating cells that express the marker and a cross indicating ablated cell descendants (in green). Lineages are shown with the same colour code as in Fig. 1. Numbers are: number of embryos with similar expression as the image/total number of embryos analysed. In all cases, the expression on the left side of the experimental embryos was positive. (A) Ci-Otx: open arrowheads indicate A7.4 blastomeres and the filled arrowhead indicates an A7.8 blastomere expressing Ci-Otx. Other expression domains are the a-line neural lineage and anterior epidermis. The insert is a control cleaving embryo. (B) Ci-Snail: 3/43 of ablated embryos showed equal expression, presumably due to failure of ablation. (C) Ci-Delta2: 1/37 of b6.5 ablated embryos showed equal expression on both sides, including b-line, probably due to failure of ablation, and was not included in the analysis. In some embryos (middle and left panel), expression can also be detected in the trunk lateral cell precursor situated between the b-line cells and the lateral A-line neural precursors. (D) Ci-ETR: among the b6.5 ablated embryos showing symmetrical expression, 5/10 had strong expression in A8.16 on both sides and 5 had weak expression on both sides. Arrowhead indicates stronger expression of Ci-ETR in A8.16 on the ablated side.

 


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Fig. 3. Ci-Nodal expression in the b6.5 blastomere depends on MEK and Ci-FGF9/16/20. Orientation and treatment of the embryo are shown on the bottom and top of each panel, respectively. (A) Expression of Ci-Nodal at the 32-cell stage. Expression in vegetal cells is variable. For the U0126 experiments, 18/20 control embryos had expression of Ci-Nodal in b6.5 and 16/20 in one or more vegetal blastomeres (average number of cells=3.9), whereas U0126-treated embryos expressed Ci-Nodal in b6.5 in 0/34 of cases and in one or more vegetal cells in 20/34 cases (average number of cells=2.3). For the FGF9/16/20-MO experiments, 26/29 control embryos had expression in b6.5 and 20/29 in one or more vegetal cells (average number of cells=2.2) whereas in FGF9/16/20-MO injected embryos 0/30 of cases expressed Ci-Nodal in b6.5 and 11/30 of cases in one or more vegetal cells (average number of cells=0.9). (B) Expression of Ci-Nodal at the early gastrula stage. 46/46 control embryos were positive; 0/46 embryos treated with U0126 at the early 32-cell stage were positive; and 40/50 embryos treated with U0126 from the late 32-cell stage were positive.

 


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Fig. 4. Nodal signalling is required for lateral neural marker gene expression, but not for neural fate. Embryo treatment is shown at the top of the panels and the marker analysed to the left of the panels. (A) Expression of Ci-Snail, Ci-Delta2 and Ci-ETR at the gastrula stage. All embryos are of vegetal pole view. Inserts show lateral view of Ci-Delta2 expression in b-line cells (weak in b8.19 and b8.17 and strong in b8.20 and b8.18). (B) Each embryo is shown in two orientations, a neural plate view (left) and a lateral view (right). A-line and a-line parts of the neural plate are indicated on the control embryos. Neural plate cells of SB431542-treated embryos were not always as well aligned as the embryos shown in the picture for Ci-Otx staining. Sometimes, cells in neighbouring rows intercalated with each other, which resulted in wider rows of 1-2 cells in depth.

 


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Fig. 7. Expression of marker genes of the medial neural plate in embryos in which Nodal signalling has been inhibited. (A) Experimental conditions are shown at the top of the panels and the marker analysed is indicated on the left of the panels. Neural plate views are shown for all embryos except cytochalasin B-treated embryos (lower panels). Insert shows a control cleaving embryo for Ci-Otx expression. Open arrowheads indicate A7.8 blastomeres. In all panels, filled arrowheads indicate ectopic marker expression. Ci-Otx expression was observed in one or both A7.8 blastomeres in 0/80 control embryos, 64/67 SB431542-treated embryos, 13/15 Ci-tALK4/5/7 injected embryos and 18/20 Ci-Nodal-MO injected embryos. (B) The marker analysed is indicated at the top of each graph. Graphs show the percentage of embryos (y-axis) that express the marker genes in 0-8 neural plate cells (x-axis). Control embryos (n=more than 100) are indicated by red bars; SB431542-treated embryos (n=82 for Ci-HB9/MNX and n=59 for Ci-FGF9/16/20) by blue horizontal striped bars; Ci-tALK4/5/7 injected embryos (n=37 for Ci-HB9/MNX and n=11 for Ci-FGF9/16/20) by blue diagonal striped bars; and Ci-Nodal-MO-injected embryos (n=24 for Ci-HB9/MNX and n=31 for Ci-FGF9/16/20) by filled blue bars.

 


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Fig. 8. A summary of the results. Embryonic stage is indicated on the left of the schematic drawings. On the left half of each drawing, the neural lineages are indicated using the same colour code as in Fig. 1. On the right half of each drawing, blastomeres expressing the markers indicated on the far right are shown in grey. From the early gastrula stage, drawings are shown in two columns; those on the right indicate control embryos and those on the left represent embryos in which Nodal signalling has been inhibited. Thick blue arrows represent signalling between blastomeres.

 





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